I'd recommened NOT to get down. Synthetic, while heavier and more bulky, isn't going to fail if they get damp, and believe me, kids aren't particularly careful with gear. Much easier to wash too.
Ours started off with Black Wolf -5 rated ones, and they also have a Caribee Hyperlite +12 rated one for summer (much lighter and smaller)
http://www.ebay.com.au/itm/CARIBEE-PLAS ... 4d0ad0e8ba and a PlasmaLite +7 deg bag
http://www.ebay.com.au/itm/CARIBEE-PLAS ... 5737103529As we've started to do more multi overnight walks and want to go lighter, I've now got a Zpacks hoodless quilt for my 12 yo son, for when he's with us (not for scout camps!), that can double as a hammock quilt 20F rated (so approx -3 C)
15 year old daughter has upgraded to a Mont Bell Super Spiral - ie they are now using adult quilts
We use the inexpensive Vietnamese techsilk liners you can get off e-bay for less than $20
We've found these to be very good - much better than paying over $60 for one in Australia
http://www.ebay.com.au/itm/Single-Pure- ... 3365204245I came across these when I was researching hammock quilts
Enlighhtened Equipment
Not sure what your budget is, but these appear to be well made hoodless sleeping bags/quilts made specifically for children that are lightweight
http://www.enlightenedequipment.com/pro ... ens-sizes/Which type depends on the height and age of the child
***Remember - you're the one that's got to carry this stuff, as well as your own gear!***If I had known about these before, in retrospect I would have bought quilt style for my kids, as they are a lot more versatile than an enclosed sleeping bag. Could open up when friends sleep over, spare bed etc etc. And what child sleeps with the hood over them properly? Quilt and beanie (and preferably one with 'ear' flaps that tie down so it doesn't come off). If their head is warm, and they have on good woollen socks and some decent thermals they'll be toasty as.
I do know some people with small kids that have used a double bag. Use it with your partner when the kids aren't along